1. Field of the Invention
The invention relates to a method and apparatus for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field, in which
after the application of an excitation radio-frequency pulse (RF-pulse) PA1 magnetic field read gradient pulses having alternate positive and negative polarity are applied for generating a plurality of magnetic resonance signals and a first set of signal samples of said magnetic resonance signals is measured, PA1 such that the signal samples from the first set are located on lines running to and fro in k-space, the lines having a mutual distance in a direction substantially perpendicular to the direction of the lines, and also PA1 a second set of signal samples is measured with both polarities of the read gradient and from the second set a set of phase errors is determined, and PA1 by means of transformation and correction for phase errors, an image is formed from the first set of signal samples.
2. Description of the Related Art
Such a method is known from DE-A 40 05 675 and from the article "Image reconstruction for echo planar imaging with nonequidistant k-space sampling", by H. Bruder et al. in Magn. Reson. Med., Vol. 23, pages 311-323 (1992). In the known method, the first set of data is acquired with continuously switched on phase encoding gradient and alternating read gradient magnetic fields. Consequently, the acquired signal samples are located on slightly curved lines substantially extending in the read direction in k-space and spaced apart in the phase encoding direction. The lines cover substantially the whole of k-space. The signal samples of the second set are acquired without a phase encoding gradient, so all samples in this second set are located on a single line in the read direction, through the origin of k-space. The known method provides only a possibility to correct phase errors that are constant in the phase encoding direction. Errors that have also a varying effect in a direction perpendicular to the read direction cannot be corrected.
In U.S. Pat. No. 5,055,789 a method and device are described for correction of phase errors in two dimensions. According to that known method a measurement is made of inhomogeneities .DELTA.H(x,y) in the stationary magnetic field and of the spatial non-linearity and the temporal variations of the gradient magnetic fields. These measurements are made beforehand by direct or indirect measurements of the magnetic fields. After data acquisition the signal samples acquired with a positive and with negative polarity read gradient are separated and image reconstruction is performed for both classes of data. Both images are separately corrected in accordance with the measured deviations of the magnetic fields and other relevant contributions. Finally the two images are synthesized into a single image.
The purpose of such measurements is to improve the quality of the magnetic resonance image to be obtained by correcting for the differences of sampled magnetic resonance signals that are acquired with a positive and a negative polarity read gradient.
In the second known method the deviations of the magnetic fields are evaluated by means of a time consuming and complicated measurement, involving the introduction in the stationary magnetic field of induction coils, phantoms and the like and deducing from that measurement the effects on magnetic resonance signals. During such a measurement the magnetic resonance apparatus cannot be used for clinical purposes. In addition, any change of the magnetic fields caused by the presence of an object or patient in the MRI device cannot be taken into account. Also, any drift, for example due to varying environmental conditions, in the deviations of the magnetic fields cannot be taken into account immediately. Other instrumental effects, such as timing effects and eddy currents, will also influence the phase and frequency encoding of the measurements.